1. The Field of the Invention
The present invention relates generally to supported catalysts and methods for making and using such catalysts (e.g., in the direct synthesis of hydrogen peroxide). More particularly, the present invention relates to the manufacture of bi-metallic catalysts that have a controlled crystal face exposure.
2. The Related Technology
Transition metal (e.g., noble metal) catalysts play a very important role in numerous industrial chemical processes, including pharmaceuticals manufacturing, petroleum refining, and chemical synthesis, among others. Cost pressures and the need for improved synthesis routes have led to continued improvement in catalyst performance.
Transition metal catalysts are typically small metal particles or crystallites. Since catalyst performance generally increases with decreased particle size, great efforts have been made to obtain particle catalysts with very small particle sizes. Recently, particle sizes of less than 10 nm have been achieved for some catalysts.
Although particle size is important to catalyst performance, there are many other factors that affect catalyst performance. One important performance characteristic of particle catalysts is selectivity. Many particle catalysts are inherently capable of catalyzing more than one reaction for a given reaction mixture. In most cases, only one product is desired and any other reaction products are by-products that reduce the yield of the manufacturing process. In addition to reducing yield, by-products may also increase the cost of isolating or concentrating the intended product.
In many cases, the different reactions are catalyzed by distinct active sites on the catalyst particle. Catalysis with a particle catalyst is achieved as reactants bond with catalyst atoms at the surface of the particle. The arrangement of the exposed atoms may determine catalytic properties of the catalyst. While one crystal face exposure may catalyze a desired reaction, another crystal face exposure may catalyze an undesired reaction.
Catalysts used for the direct synthesis of hydrogen peroxide exemplify catalysts in which selectivity is greatly affected by crystal face exposure. Direct synthesis of hydrogen peroxide is currently performed using palladium and platinum particles dispersed on a support material. Catalyst surfaces that have a (110) type crystal face exposure favor the formation of hydrogen peroxide, while catalyst surfaces with a (111) type crystal face exposure favor the formation of water, which is thermodynamically favored over the less stable hydrogen peroxide. Consequently, catalyst particles with predominantly (110) crystal face exposure are preferred for direct synthesis of hydrogen peroxide.
Recently, manufacturing techniques have been developed that allow catalysts particles to be formed with a controlled crystal face exposure. Examples of supported nanocatalysts are disclosed in U.S. Pat. Nos. 7,045,479 and 7,011,807. These catalyst particles can be manufactured using an organic control agent. The control agent molecules are reacted with catalyst atoms in solution to form organometallic complexes. The complexed atoms are then allowed or caused to form particles. As the particles form, the control agent molecules influence the crystal face exposure. Particles formed using this method have shown dramatic improvements in selectivity, reduced particle size, and improved particle stability.
While these recent improvements in catalyst performance have been substantial, there is still a need for improved selectivity. In particular, there is a need to improve the selectivity of bi-metallic catalysts. It is often essential to include more than one metal in a particle catalyst. The purpose or advantage of including more than one metal differs between catalysts, but in many cases the second metal is provided to enhance the rate of reaction, improve selectivity, and/or prevent catalyst poisoning. However, controlling crystal face exposure for bi-metallic catalysts can be more difficult than for a pure metal due to the interactions between the different catalyst atoms.